Particle accelerating tubes



Feb. 14, 1967 w, ALLEN ETAL 3,304,454

PARTICLE ACCELERATING TUBES Filed Aug. 5, 1962 2 Sheets-Sheet 1 Feb. 14,1967 w. D. ALLEN ETAL 3,304,454

PARTICLE ACCELERATING TUBES Filed Aug. 3, 1962 2 Sheets-Sheet 2 UnitedStates Patent 3,304,454 PARTICLE ACCELERATING TUBES William DouglasAlien, Abingdon, Albert Edward Pyrah, Harwell, and John Henry Partridge,Upton, England, assignors to National Institute for Research in NuclearScience, Rutherford High Energy Laboratory, Harwell, Berkshire, andUnited Kingdom Atomic Energy Authority, London, England Filed Aug. 3,1962, Ser. No. 214,748 Claims priority, application Great Britain, Aug.11, 1961, 29,159/ 61 1 Claim. (Cl. 31363) This invention relates toaccelerating tubes suitable for use with electrostatic generators, e.g.of the Van de Graafi type.

The performance of an electrostatic generator as a particle acceleratoris normally limited by the quality of the accelerating tube, at highvacuum, through which the particles (positive or negative ions) areaccelerated. The limitation manifests itself through the X-raysgenerated by the electrons which stream back to the (positive) hightension electrode through the evacuated region. The X-rays, which can bereadily detected outside the pressure vessel enclosing the generator,cause high ionisation currents in the high pressure gas in the vessel,which constitutes the insulator against high voltage breakdown. As boththe electron currents and the concomitant X-rays rise steeply above agiven voltage threshold, machine performance is commonly limited by theionization currents liberated by the X-rays in the gas of the pressurevessel.

The origin of these electron currents is obscure. The tube is commonlydesigned with say 150 electrodes separated by glass rings 1" thick. Theelectrodes are commonly plates or dishes in which the planar regionadjacent to the beam (which passes through a hole in the plate or dish)is normal to the direction of the beam. Two electrodes separated by aglass ring in vacuum will withstand up to 200 kv., so that, if thestrength of the system were linear, the composite tube should withstanda voltage of 30 mv. In practice, however, such a tube would be unlikelyto withstand more than mv. without heavy electron loading setting in. Itis clear that some multiplication mechanism (or mechanisms) is at work,but the phenomenon has not been elucidated.

According to the present invention an accelerating tube comprises aplurality of electrodes whereof the planar region adjacent the beam isnormal to the tube axis followed by a plurality of electrodes whereofthe planar region adjacent the beam is tilted relative to a plane normalto the tube axis, the azimuthal direction of tilt of successiveelectrodes being incrementally varied to form at least one pair ofconsecutive 360 spirals spiralling in opposite directions, and theincrements being such that the net displacement and angular deviation ofthe beam in traversing the pair of spirals are substantially zero.

To enable the nature of the present invention to be more readilyunderstood, attention is directed, by way of example, to theaccompanying drawings wherein:

FIG. 1 is a sectional elevation of a tilted electrode.

FIG. 2 is a plan view of the electrode of FIG. 1.

FIG. 3 is a simplified sectional elevation of an accelerating tubeembodying the invention.

FIG. 4 is a sectional elevation of a modified tilted electrode.

As already stated, it is common practice in accelerating tubes to useelectrodes in the form of plates or dishes in which the planar areaadjacent to the beam is normal to the beam. The present inventioninvolves the use of electrodes in which this planar area makes an anglewith the plane normal to the tube axis, and a dished electrode of thiskind is shown in FIGS. 1 and 2. It will be seen "ice that the planararea 1 adjacent the hole 2, through which the beam passes, makes anangle 6 with the normal to the axis of the beam, which is indicated bythe arrow 4. The two additional holes 3 are provided to make the tubeeasier to pump out. The direction of tilt of this electrode is indicatedby the arrow 5 in FIG. 2.

If a succession of the electrodes shown in FIG. 1 is used, the beam willexperience a sideways deflection which is not subject to latercorrection. This sideways deflection is a maximum at low beamvelocities; if the beam is travelling at high velocity, the time that itspends in the vicinity of two electrodes is small, and the deflectionexperienced is correspondingly low. The same is true of whateverparticles are responsible for the multiplication mechanism of thephenomenon of electron loading; if the electrodes where they originateare tilted with respect to the beam, as in FIG. 1, they will experiencea sideways deflection which will persist throughout subsequentacceleration and will cause them to strike electrodes further along thetube with energies far less than they would attain by accelerationthroughout the length of the machine. Since the X-ray yield andpenetration increase rapidly with increasing electron energy, theloading produced by such X-rays will be very greatly reduced, if notaltogether suppressed.

Equally, however, a succession of electrodes tilted at a constant anglethroughout an accelerating tube would rapidly deflect the desiredparticle beam until it hit the electrodes. Now, as already indicated,the effect of loading is by no means linear, being small for shortlengths of tube, and the effect of the deflection produced by tiltedelectrodes is small for particles which have attained a high velocity.The present accelerating tube takes ad vantage of these efliects in themanner shown inFIG. 3, in which the successive electrodes are numberedE1, E2, etc., from the input end of the tube. At the input (highvoltage) end, where the particle velocity is low, a section of ordinarytube is used consisting of 29 electrodes normal to the axis of the tube.In this section the particles are accelerated to about 1.5 mv. Fromelectrode 30 onwards, however, the tilted electrodes shown in FIGS. 1and 2 are used, the angle 0 being 10". If all the tilted electrodes wereparallel, the resultant deviation of the beam from the tube axis wouldnot be negligible. To avoid this deviation the azimuthal direction ofthe tilt of successive electrodes is incrementally varied, i.e. theelectrodes are spiralled, so that the direction of deflection is also aspiral. If the azimuthal direction of tilt of electrode 30 is taken asa=0 (the datum), then with a suitable rate of change of a betweensuccessive electrodes the net deflection of the beam can be madenegligible at electrode 45 where 0;:360". FIG. 2 indicates the positionof the arrow 5 for an electrode whose azimuthal direction of tilt is (1relative to the datum.

There must be taken into account, however, not only the deflection,i.e., the angular deviation of the particle beam from the original axis,but its displacement from that axis. It can be shown that for the63-electrode tube of FIG. 3 this displacement is quite large at electrode 45 (about 0.1 inch, in the aximuthal direction of a= Thedisplacement can be corrected by using a second spiral spiralling in theopposite direction. Thus in FIG. 3 the first spiral ends at electrode 45where a=360; the second spiral begins at electrode 45 and ends atelectrode 63 where oc=36(). By suitably varying the rate of change of abetween successive electrodes as hereinafter described, the netdisplacement as well as the net deviation can be made negligibly small.

Since the velocity of the particles increases as they traverse eachspiral, it follows that the angular increments between successiveelectrodes require to be greater at The arrow 6 in the beginning than atthe end of each spiral. Let it be assumed that the electrodes areequi-spaced along the tube (so that the number of each electrode is ameasure of its distance from the input end), that the first spiralstarts at electrode n and ends at electrode 11 and that the second(reverse) spiral starts at electrode in and ends at electrode 11 Then,for the net deviation in the first spiral to be negligible it can beshown that a mam) va-m where a and for the net deviation in the second(reverse) spiral to be negligible that (x D a where a =0. Similarly, fornegligible displacement of the beam it can be shown that The followingtable gives values of a for the 63-electrode tube shown in FIG. 3, whichmeet the above conditions. In this design 6:10 and the hole 2 is 1 /2inches in diameter.

Electrode a Electrode 04 Electrode 01 No. No. No.

The 63-electrode tube described with reference to FIG. 3 is specificallydesigned for a total tube voltage of about 2.5 mv. (i.e. 40 kv. perelectrode), the electrodes being spaced 1 inch apart. In the generalcase various parameters have to be optimised, the factors affectingthese parameters being:

(a) The acceleration which electrons emitted from the electrodes mayreceive before striking an electrode further up the tube.

(b) The distance by which the particle beam may be displaced from thebeam axis after the first spiral. Clearly this displacement must be suchthat, after taking account of structural tolerances, the cross-sectionaldiameter of the beam etc., interception of the beam by the electrodes ishighly improbable.

Taking these factors into account, two parameters which are to a certainextent arbitrary are:

(i) The angle of tilt, 0, of the individual electrodes. It can be shownthat in the described design with a hole diameter of 1 inches and 0:10",the probably maximum electron energy is about 500 kv. With a smallervalue of 0 the maximum electron energy would be greater,

and witlf a greater angle the beam displacement after 4 the first spiralwould be greater. The value chosen, 10, represents a reasonablecompromise.

(ii) The mean rate of variation of the angle a. If this rate were tooslow, part of the beam might be intercepted by the electrodes, e.g. inthe example of FIG. 3 if one complete spiral (360) occupied all 60electrodes, it can be shown that the displacement of the particle beamwould amount to about 0.4 inch. This estimate is naturally affected bythe overall length of the tube and the applied voltage; as the velocityof the beam particles becomes greater the beam deviation diminishes andhence also the total displacement. For a long tube, therefore, theoptimum compromise is to choose a rate of spiralling which is high wherethe particle velocity is low and vice versa. Thus for a long tubeseveral pairs of spirals having diiferent mean spiralling rates may benecessary.

In cases where the electrode size does not permit the machining of thepumping holes 3 (FIG. 1), adequate pumping speed can be combined withelectron suppression by increasing the diameter of the hole 2 andolfsetting it from the beam axis 4 towards the lower side of theelectrode. FIG. 4 shows such an electrode in which the hole 2' isincreased to 3 inches diameter and is offset so that the distance fromthe beam to the nearest point 7 on the circumference of the hole isinch. Electrons leaving this electrode at the point 7 will traverse thetube across the axis and strike an electrode at the point further up thetube where the direction of tilt is reversed, i.e. a=180. The overalleffect is then equivalent to tube of electrodes having coaxial 1 /2 inchdiameter holes. To limited extent this effect depends on the rate ofspiralling and is linked with the factor (b) already considered. Forexample, if the spiral is too fast, the electrons may miss the electrodeedge after the first half-spiral, and travel appreciably further,thereby generating X-rays of higher energy. However in practical casesthe rate of spiralling coincides with the rate required if the electronsare to strike the electrodes at the end of the first half-spiral.

We claim:

An accelerating tube for an electrostatic generator comprising aplurality of electrodes whereof the planar region adjacent the beam isnormal to the tube axis followed by a plurality of electrodes whereofthe planar region adjacent the beam is tilted relative to a plane normalto the tube axis, the azimuthal direction of tilt of successiveelectrodes being incrementally varied to form at least one pair ofconsecutive 360 spirals spiralling in opposite directions, and theincrements being such that the net displacement and angular deviation ofthe beam in traversing the pair of spirals are substantially zero.

References Cited by the Examiner UNITED STATES PATENTS 1/1940 Van deGraatf 313-63 3/1941 Heil 328256 X

